Analysis and Reduction of False Alarms at LASA

Abstract

Abstract : The authors show that many of the 'false alarms' which required analyst intervention in the preparation of the LASA event summary were due to local or regional events. Analyses showed that these false alarms occur predominantly on weekdays during local working hours, suggesting that the seismic events are of man-made origin. The false alarm rate decreases on weekends and holidays, and LASA reports more teleseismic events. To reduce the number fo false alarms it is necessary to steer detection beams to local areas. By detecting local events on these beams and by using a higher S/N threshold in processing these signals, the number of false alarms can be effectively reduced from the original 57% to 41%. A new beam set has been developed and deployed which concentrates teleseismic beams in high seismicity areas instead of spacing them equidistantly apart. This arrangement reduced the average detection errors from 200 km to 50 km, there is also some indication of a lowered detection threshold on the order of 0.1 + or - 0.1 magnitude units.